Polyurethane

ABSTRACT

A polyurethane obtainable by reacting a polyisocyanate, a polyester formed from a dimer fatty acid and/or dimer fatty diol, and a chain extender, wherein the polyester is additionally formed from a 1,4;3,6 dianhydrohexitol and/or the chain extender comprises a 1,4;3,6 dianhydrohexitol. The polyurethane is particularly suitable for use as a hotmelt adhesive.

FIELD OF INVENTION

The present invention relates to a polyurethane, a process of making thepolyurethane, and in particular to the use thereof as a hotmeltadhesive.

BACKGROUND

Polyurethanes are extremely versatile materials and have been used in awide variety of applications such as foam insulation, car seats,abrasion resistant coatings, and adhesives, particularly hotmeltadhesives.

Hotmelt adhesives are adhesives which are solid at room temperature andwhich are applied in the form of a melt, usually at temperatures in therange from 80 to 250° C. Cooling of the melt results in rapid physicalsetting of the adhesive. Some hotmelt adhesives, such as polyurethanes,can subsequently undergo a chemical reaction of functional groupspresent in the adhesive with moisture to form a crosslinked, infusibleadhesive. It is only after this chemical curing with moisture,accompanied by an increase in the size of the molecule or crosslinking,that the adhesive acquires its final properties. The initial bondstrength of the adhesive, ie before cure, is referred to as the greenstrength of the adhesive.

Hotmelt adhesives can be used to adhere a wide range of materials, suchas polar substrates like paper, wood and metal, and low-energysubstrates such as polyolefins. An obvious benefit is the absence of anysolvent, which makes hot melt adhesives a technology of increasingimportance.

A wide range of materials have been used as hotmelt adhesives, such aspolyamides, polyesters and copolymers thereof, as well as polyurethanes.Polyurethane hotmelt adhesives have certain advantages over othermaterials, such as versatility in use due to low melting temperature,and good mechanical properties after curing has taken place.

Polyurethane hotmelt adhesives with high green strength are known, suchas polyurethanes containing crystalline polyester polyols, but thesenormally suffer from other problems such as high crystallinity whichresults in brittleness or lack of flexibility. Alternatively,polyurethanes containing polyether may have good flexibility, butgenerally also have low green strength.

It is this combination of both high green strength, good adhesion andgood flexibility that has been difficult to achieve with polyurethanehotmelt adhesives. Other desirable properties include thermal andhydrolytic stability, and adhesion to low-energy substrates.

REVIEW OF THE PRIOR ART

U.S. Pat. No. 3,933,705 discloses the use of a 36 carbon dimerised fattyacid as a fatty modifier in rapid-setting polyurethanes.

U.S. Pat. No. 4,443,563 is directed to the use of 1,4:3,6dianhydrohexitols, such as isosorbide, in polyurethane.

U.S. Pat. No. 5,994,493 is directed to the use of a polyurethane formedfrom an aromatic dihydroxy compound, such as bisphenol A, as a hotmeltadhesive.

SUMMARY OF THE INVENTION

We have now surprisingly discovered a polyurethane which can be used asa hot-melt adhesive, which reduces or substantially overcomes at leastone of the aforementioned problems.

Accordingly, the present invention provides a polyurethane obtainable byreacting a polyisocyanate, a polyester formed from a dimer fatty acidand/or dimer fatty diol, and a chain extender, wherein the polyester isadditionally formed from a 1,4:3,6 dianhydrohexitol and/or the chainextender comprises a 1,4:3,6 dianhydrohexitol.

The invention also provides a process for preparing a polyurethane whichcomprises (i) reacting a polyisocyanate with a polyester formed from adimer fatty acid and/or dimer fatty diol, to form anisocyanate-terminated prepolymer, and (ii) reacting the prepolymer witha chain extender, wherein the polyester is additionally formed from a1,4:3,6 dianhydrohexitol and/or the chain extender comprises a 1,4:3,6dianhydrohexitol.

The invention further provides a hotmelt adhesive comprising apolyurethane obtainable by reacting a polyisocyanate, a polyester formedfrom a dimer fatty acid and/or dimer fatty diol, and a chain extender,wherein the polyester is additionally formed from a 1,4:3,6dianhydrohexitol and/or the chain extender comprises a 1,4:3,6dianhydrohexitol.

The polyester used in the present invention is formed from, ie comprisesthe reaction product of, at least one dimer fatty acid and/or dimerfatty diol and/or equivalent thereof. Polyester is normally produced ina condensation reaction between at least one polycarboxylic acid and atleast one polyol. Dicarboxylic acids and diols are preferred. Thepreferred dicarboxylic acid component of the polyester used in thepresent invention comprises at least one dimer fatty acid.

The term dimer fatty acid is well known in the art and refers to thedimerisation product of mono- or polyunsaturated fatty acids and/oresters thereof. Preferred dimer fatty acids are dimers of C₁₀ to C₃₀,more preferably C₁₂ to C₂₄, particularly C₁₄ to C₂₂, and especially C₁₈alkyl chains. Suitable dimer fatty acids include the dimerisationproducts of oleic acid, linoleic acid, linolenic acid, palmitoleic acid,and elaidic acid. The dimerisation products of the unsaturated fattyacid mixtures obtained in the hydrolysis of natural fats and oils, e.g.sunflower oil, soybean oil, olive oil, rapeseed oil, cottonseed oil andtall oil, may also be used. Hydrogenated, for example by using a nickelcatalyst, dimer fatty acids may also be employed.

In addition to the dimer fatty acids, dimerisation usually results invarying amounts of oligomeric fatty acids (so-called “trimer”) andresidues of monomeric fatty acids (so-called “monomer”), or estersthereof, being present. The amount of monomer can, for example, bereduced by distillation. Particularly preferred dimer fatty acids, usedto form the polyester component of the polyurethane according to thepresent invention, have a dicarboxylic (or dimer) content of greaterthan 45%, more preferably greater than 60%, particularly greater than70%, and especially greater than 75% by weight. The trimer content ispreferably less than 55%, more preferably in the range from 5 to 40%,particularly 10 to 30%, and especially 15 to 25% by weight. The monomercontent is preferably less than 10%, more preferably in the range from0.5 to 5%, particularly 1 to 4%, and especially 2 to 3% by weight. Allof the above % by weight values are based on the total weight of trimer,dimer and monomer present.

The dicarboxylic acid component of the polyester preferably alsocomprises non-dimeric dicarboxylic acids (hereinafter referred to asnon-dimeric acids). The non-dimeric acids may be aliphatic or aromatic,and include dicarboxylic acids and the esters, preferably alkyl esters,thereof, preferably linear dicarboxylic acids having terminal carboxylgroups having a carbon chain in the range from 2 to 20, more preferably6 to 12 carbon atoms, such as adipic acid, glutaric acid, succinic acid,pimelic acid, suberic acid, azelaic acid, sebacic acid, heptanedicarboxylic acid, octane dicarboxylic acid, nonane dicarboxylic acid,decane dicarboxylic acid, undecane dicarboxylic acid, dodecanedicarboxylic acid and higher homologs thereof. Adipic acid isparticularly preferred.

A monomeric dicarboxylic acid anhydride, such as phthalic anhydride, mayalso be employed as the or as part of the non-dimeric acid component.

The polyester is preferably formed from dimer fatty acids to non-dimeracids present at a weight ratio in the range from 10 to 100:0 to 90%,more preferably 30 to 70:30 to 70%, particularly 40 to 60:40 to 60%, andespecially 45 to 55:45 to 55% by weight of the total dicarboxylic acids.

The polyol component of the polyester used in the present invention issuitably of low molecular weight, preferably in the range from 50 to650, more preferably 70 to 200, and particularly 100 to 150. The polyolcomponent may comprise polyols such as pentaerythritol, triols such asglycerol and trimethylolpropane, and preferably diols. Suitable diolsinclude straight chain aliphatic diols such as ethylene glycol,diethylene glycol, 1,3-propylene glycol, dipropylene glycol,1,4-butylene glycol, 1,6-hexylene glycol, branched diols such asneopentyl glycol, 3-methyl pentane glycol, 1,2-propylene glycol, andcyclic diols such as 1,4-bis(hydroxymethyl)cyclohexane and(1,4-cyclohexane-dimethanol). 1,4-butylene glycol and/or 1,6-hexyleneglycol are preferred, and 1,6-hexylene glycol is a particularlypreferred diol.

The polyol component may also comprise a dimer fatty diol. Dimer fattyacids are mentioned above in relation to the dicarboxylic acidcomponent, and dimer fatty diols can be produced by hydrogenation of thecorresponding dimer fatty acid. The same preferences above for the dimerfatty acid apply to the corresponding dimer fatty diol component of thepolyester.

The polyol component may also comprise a 1,4:3,6 dianhydrohexitol.Preferred 1,4:3,6 dianhydrohexitols are mannitol, sorbitol and iditol,which are commonly known as isomannide, isosorbide and isoidide afterthe relevant parent hexitol. Isosorbide (or 1,4:3,6dianhydro-D-sorbitol) is particularly preferred. Isosorbide can beconveniently made from renewable resources such as sugars and starches,for example from D-glucose by hydrogenation followed by acid catalyseddehydration.

The polyester is preferably formed from dicarboxylic acid to diolstarting materials at a molar ratio in the range from 1:1.0 to 5.0, morepreferably 1:1.2 to 3.0, particularly 1:1.4 to 2.0, and especially 1:1.5to 1.7. Thus, the diol is preferably present in molar excess so as toobtain polyester terminated at both ends with OH groups.

In a preferred embodiment, the polyester is formed from, ie comprisesthe reaction products of, dimer fatty acid, adipic acid, and1,6-hexylene glycol, preferably at a molar ratio in the range from 0.01to 1:0.1 to 1:1, more preferably 0.05 to 0.75:0.2 to 0.75:1,particularly 0.1 to 0.2:0.4 to 0.6:1, and especially approximately0.14:0.5:1.

The polyester preferably has a molecular weight (number average) in therange from 1,000 to 6,000, more preferably 1,700 to 3,000, particularly1,800 to 2,500, and especially 1,900 to 2,200.

The polyester preferably has a glass transition temperature (Tg) in therange from −60 to 0° C., more preferably −50 to −5° C., particularly −40to −10° C., and especially −35 to −15° C.

The polyester preferably has a hydroxyl value (measured as describedherein) in the range from 10 to 100, more preferably 30 to 80,particularly 40 to 70, and especially 50 to 60 mgKOH/g. In addition, thepolyester preferably has an acid value (measured as described herein) ofless than 2, more preferably less than 1.5, particularly less than 1.0,and especially less than 0.6.

The polyisocyanate component is preferably at least one isocyanate whichhas a functionality of at least 2, and may be an aliphatic isocyanatesuch as hexamethylene 1,6-diisocyanate, but more preferably is anaromatic isocyanate such as tolylene diisocyanate, m-phenylenediisocyanate, p-phenylene diisocyanate, xylylene diisocyanate,4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate,isophorone diisocyanate, polymethylenepolyphenyl diisocyanate,3,3′-dimethyl-4,4′-biphenylene diisocyanate,3,3′-dimethyl-4,4′-diphenylmethane diisocyanate,3,3-dichloro-4,4′-biphenylene diisocyanate, 1,5-naphthalenediisocyanate, or modified compounds thereof such as uretonimine-modifiedcompounds thereof. The polyisocyanate monomers can be used alone or asmixtures thereof. In a preferred embodiment, 4,4′-diphenylmethanediisocyanate (MDI) is used, more preferably as the only polyisocyanatesemployed.

In one embodiment of the invention, at least one of the aforementionedpolyisocyanates is reacted with at least one of the aforementionedpolyesters, to form a prepolymer.

The molar ratio of polyisocyanate to polyester starting materials whichare mixed together to react to form the prepolymer is preferably in therange from 20 to 80:20 to 80%, more preferably 35 to 75:25 to 65%,particularly 45 to 70:30 to 55%, and especially 55 to 70:30 to 45%. Thepolyisocyanate is preferably used in molar excess relative to OH groupcontent of the polyester, so as to obtain a reactive hotmelt containingisocyanate-terminated prepolymer and sufficient unreactedpolyisocyanate, such that later addition of the chain extender canresult in reaction to form the polyurethane, without the requirement foradding further polyisocyanate.

The prepolymer reaction mixture preferably has an isocyanate content(measured as described herein) in the range from 2 to 10%, morepreferably 2.5 to 8%, particularly 3 to 6.5%, and especially 3.5 to 5.5%NCO.

The chain extender component used to form the polyurethane according tothe present invention suitably comprises a low molecular compound having2 or more active hydrogen groups, for example polyols such as ethyleneglycol, diethylene glycol, propylene glycol, 1,4-butylene glycol,1,5-pentylene glycol, methylpentanediol, 1,6-hexylene glycol, neopentylglycol, trimethylolpropane, hydroquinone ether alkoxylate, resorcinolether alkoxylate, glycerol, pentaerythritol, diglycerol, dextrose, and a1,4:3,6 dianhydrohexitol such as isomannide, isosorbide and isoidide;aliphatic polyhydric amines such as ethylenediamine,hexamethylenediamine, and isophorone diamine; aromatic polyhydric aminessuch as methylene-bis(2-chloroaniline), methylenebis(dipropylaniline),diethyl-toluenediamine, trimethylene glycol di-p-aminobenzoate;alkanolamines such as diethanolamine, triethanolamine anddiisopropanolamine.

In a preferred embodiment of the invention, the chain extender is a1,4:3,6 dianhydrohexitol, more preferably isomannide, isosorbide and/orisoidide. Isosorbide is particularly preferred.

In one embodiment of the invention, at least one of the aforementionedpolyesters is added together with the chain extender to react with atleast one of the aforementioned prepolymers in order to form thepolyurethane. The polyester employed may be the same as or different tothe polyester used to form the prepolymer.

The molar ratio of chain extender to total polyester (includingpolyester present in the prepolymer) employed is preferably in the rangefrom 0.1 to 10:1, more preferably 0.2 to 8:1, particularly 0.3 to 5:1,and especially 0.5 to 1.5:1.

The dimer fatty acid and/or dimer fatty diol content of the polyurethaneis preferably in the range from 5 to 50%, more preferably 10 to 40%,particularly 15 to 35%, and especially 20 to 30% by weight.

The 1,4:3,6 dianhydrohexitol, preferably isosorbide, content of thepolyurethane is preferably in the range from 1 to 20%, more preferably 2to 10%, particularly 3 to 7%, and especially 3 to 5% by weight.

The molar ratio of 1,4:3,6 dianhydrohexitol, preferably isosorbide, topolyester in the polyurethane is preferably in the range from 0.2 to2.5:1, more preferably 0.5 to 1.5:1, particularly 0.6 to 1:1, andespecially 0.65 to 0.8:1.

In the present invention, the chain extender composition may optionallycontain other additives such as urethane promoting catalysts,surfactants, stabilizers and pigments.

Suitable catalysts are the normal polyurethane catalysts such ascompounds of divalent and tetravalent tin, more particularly thedicarboxylates of divalent tin and the dialkyl tin dicarboxylates anddialkoxylates. Examples include dibutyl tin dilaurate, dibutyl tindiacetate, dioctyl tin diacetate, dibutyl tin maleate, tin(II) octoate,tin(II) phenolate, and the acetyl acetonates of divalent and tetravalenttin. In addition, tertiary amines or amidines may also be employed,either alone or in combination with the aforementioned tin compounds.Examples of amines include tetramethyl butane diamine,bis-(dimethylaminoethyl)-ether, 1,4-diazabicyclooctane (DABCO),1,8-diazabicyclo-(5.4.0)-undecane, 2,2′-dimorpholinodiethyl ether,dimethyl piperazine, and mixtures thereof.

Suitable surfactants include silicone surfactants such asdimethylpolysiloxane, polyoxyalkylene polyol-modifieddimethylpolysiloxane and alkylene glycol-modified dimethylpolysiloxane;and anionic surfactants such as fatty acid salts, sulfuric acid estersalts, phosphoric acid ester salts and sulfonates.

Suitable stabilizers are materials which stabilize the viscosity of thepolyurethane during its production, storage and application, and includemonofunctional carboxylic acid chlorides, monofunctonal highly reactiveisocyanates, and non-corrosive inorganic acids. Examples of suchstabilizers are benzoyl chloride, toluene sulfonyl isocyanate,phosphoric acid or phosphorous acid. In addition, suitable hydrolysisstabilizers include for example the carbodiimide type.

Suitable pigments include inorganic pigments such as transition metalsalts; organic pigments such as azo compounds; and carbon powder.

The polyurethane according to the present invention may be produced bysimple mixing of the prepolymer and chain extender, preferably at aNCO/OH ratio in the range from 1.5 to 5:1, more preferably 1.7 to 3:1.,and particularly 1.8 to 2:1.

A particular advantage of a polyurethane according to the presentinvention is that it has a green strength value (measured as describedherein) of preferably greater than 20, more preferably greater than 50,particularly greater than 100, and especially greater than 500, andgenerally up to 1,000 kPa after 1 minute; and/or preferably greater than100, more preferably greater than 200, particularly greater than 500,and especially greater than 1000, and generally up to 1,500 kPa after 5minutes; and/or preferably greater than 200, more preferably greaterthan 300, particularly greater than 500, and especially greater than1,000, and generally up to 1,500 kPa after 30 minutes.

The polyurethane suitably has a tensile strength (measured as describedherein) of greater than 20, preferably in the range from 30 to 200, morepreferably 40 to 150, particularly 45 to 100, and especially 50 to 80kgcm⁻².

The elongation at break (measured as described herein) of thepolyurethane is preferably greater than 150%, more preferably greaterthan 200%, particularly in the range from 250 to 550% and especially 300to 400%.

One particular advantage of the polyurethane described herein isimproved substrate adhesion, particularly on low energy surfaces,preferably having a surface energy of less than 50, more preferably inthe range from 10 to 45, particularly 20 to 40, and especially 25 to 35mN/m. A preferred substrate is a polyolefin such as polypropylene orpolyethylene, more preferably polyethylene. Laminate structures may beformed, for example using the polyurethane of the present invention toadhere two low surface energy materials together, preferably poyolefin.Alternatively, laminates may be formed by adhering a low surface energymaterial to a high surface energy material. The physical form of thesubstrate may vary over a wide range from thin films to 3-dimensionalobjects.

In a particularly preferred embodiment of the invention, thepolyurethane when applied as an adhesive layer on a polyolefinsubstrate, preferably polyethylene, suitably has an adhesive strength(measured as described herein) of greater than 200, preferably greaterthan 400, more preferably greater than 600, particularly greater than800, and especially greater than 1,000, and generally up to 2,000 kPa.

The polyurethane described herein may be used as an adhesive, preferablyas a hotmelt adhesive, in a wide range of applications such aswoodworking and construction, shoe manufacture, in various automotiveuses, and in inks, for example printed onto flexible packaging. Aparticularly preferred application is as an adhesive for lamination.

The hotmelt adhesives according to the invention may optionally containtackifying resins such as, for example, abietic acid, abietic acidesters, terpene resins, terpene/phenol resins or hydrocarbon resins andalso fillers (for example silicates, talcum, calcium carbonates, claysor carbon black), plasticizers such as, for example, phthalates orthixotropicizing agents (for example Bentone, pyrogenic silicas, ureaderivatives, fibrillated or pulp chopped fibers) or pigment pastes orpigments.

Alternative uses of the polyurethane according to the invention includesealants, coatings and insulation materials.

The invention is illustrated by the following non-limiting examples.

In this specification, the following test methods have been used.

(a) For Polyester

-   (i) Molecular weight (number average) was determined by end group    analysis.-   (ii) The glass transition temperature (Tg) was measured by    Differential Scanning Calorimetry (DSC) at a scan rate of 20°    C./minute using a Mettler DSC30.-   (iii) The hydroxyl value is defined as the number of mg of potassium    hydroxide equivalent to the hydroxyl content of 1 g of sample, and    was measured by acetylation followed by hydrolysation of excess    acetic anhydride. The acetic acid formed was subsequently titrated    with an ethanolic potassium hydroxide solution.-   (iv) The acid value is defined as the number of mg of potassium    hydroxide required to neutralise the free fatty acids in 1 g of    sample, and was measured by direct titration with a standard    potassium hydroxide solution.    (b) For Prepolymer-   (i) The isocyanate value is defined as the weight % content of    isocyanate in the sample and was determined by reacting with excess    dibutylamine, and back titrating with hydrochloric acid.    (c) For Polyurethane-   (i) Green strength was measured by melting the polyurethane onto a    100×25 mm low density polyethylene specimen, layer thickness 150 μm,    followed by application of a second piece of polyethylene in a lap    joint, with an overlap of 25×25 mm. Several samples (up to ten) were    placed in the oven at 55° C. and removed after five minutes.

The joined polyethylene specimens were subsequently subjected to tensiletests in a Zwick tensile tester. The maximum load applied to thespecimen before failure of the bond was recorded after specific timeintervals of 1, 5 and 30 minutes. The increase in maximum load requiredillustrates the build up of green strength.

-   (ii) Tensile strength was determined according to ISO 37/DIN 53504    using a Z82B29 sample die. The samples were conditioned for a    minimum of 24 hours, undeflected and undistorted at 23° C. and 50%    relative humidity, prior to testing.-   (iii) Elongation at break was measured according to ISO 37/DIN 53504    using Z82B29 sample die. The samples were conditioned for a minimum    of 24 hours, undeflected and undistorted at 23° C. and 50% relative    humidity, prior to testing.-   (iv) Adhesive strength was determined using a lap shear bond test    according to ASTM D1002 using an Instron tensile tester. Two pieces    of low density polyethylene of thickness 4 mm were glued together by    melting 25×25 mm of the polyurethane adhesive onto the first piece    of polyethylene, followed by application of the second piece of    polyethylene. The adhesive was allowed to set for 5 minutes at room    temperature under light pressure, and then left for 6 days    undeflected and undistorted at 23° C. and 50% relative humidity. The    load at which the adhesive bond failed is the adhesive strength of    the polyurethane to polyethylene.

EXAMPLES Example 1

800 g of Priplast 3192 (trade mark, ex Uniqema) (polyester formed fromdimer fatty acid) was charged to a reactor equipped with thermocouple,stirrer and nitrogen inlet and heated to 120° C. When the temperaturehad reached 120° C., vacuum was applied to 25 mbar to remove anyresidual water and maintained for one hour. Subsequently, the vacuum wasremoved and the temperature was lowered to 80° C., and 300 g of flakepure MDI (Desmodur 44, ex Bayer) was charged to the reactor and themixture was allowed to react for 2 hours. Prior to addition of 40 g ofisosorbide (ex Aldrich) as a chain extender, the temperature wasincreased to 120° C. One hour after addition of the isosorbide, thepolyurethane was poured into a tin.

The polyurethane was subjected to the test procedures described aboveand the results were;

-   (i) The green strength of the reactive hotmelt was 100 kPa after 1    minute, 230 kPa after 5 minutes and 350 kPa after 30 minutes.-   (ii) The tensile strength was 58 kgcm⁻².-   (iii) The elongation at break was 300%.-   (iv) The adhesive strength was 600 kPa.

Example 2

This is a comparative example not according to the invention.

The procedure of Example 1 was repeated except that 400 g of Priplast3192, 206 g of flake pure MDI and no isosorbide was used.

The polyurethane was subjected to the test procedures described aboveand the results were;

-   (i) The green strength of the reactive hotmelt was <10 kPa after 1,    5 and 30 minutes (and also after 60 minutes).-   (ii) The tensile strength was 133 kgcm-2.-   (iii) The elongation at break was 700%.-   (iv) The adhesive strength was 600 kPa.

Example 3

This is a comparative example not according to the invention.

The procedure of Example 1 was repeated except that 800 g of hexane dioladipate (Fomrez ER 196, ex Crompton) was used instead of Priplast 3192.

The polyurethane was subjected to the test procedures described aboveand the results were;

-   (i) The green strength of the reactive hotmelt was 200 kPa after 1,    5 and 30 minutes (and also after 60 minutes).-   (ii) The tensile strength was 230 kgcm⁻².-   (iii) The elongation at break was 190%.-   (iv) The adhesive strength was 200 kPa.

The above examples illustrate the improved properties of a polyurethaneaccording to the present invention.

1. A polyurethane obtainable by reacting a polyisocyanate, a polyesterformed from a dimer fatty acid and/or dimer fatty diol, and a chainextender, wherein the polyester is additionally formed from a 1,4:3,6dianhydrohexitol and/or the chain extender comprises a 1,4:3,6dianhydrohexitol.
 2. A polyurethane according to claim 1 wherein thechain extender comprises a 1,4:3,6 dianhydrohexitol, and optionally thepolyester is additionally formed from a 1,4:3,6 dianhydrohexitol.
 3. Apolyurethane according to claim 1 wherein the polyester is additionallyformed from a non-dimer acid, and preferably the ratio of dimer fattyacids to non-dimer acids is in the range from 30 to 70:30 to 70% byweight of the total dicarboxylic acids.
 4. A polyurethane according toclaim 3 wherein the non-dimer acid comprises adipic acid.
 5. Apolyurethane according to claim 1 wherein the polyester is formed fromdimer fatty acid, adipic acid and 1,6-hexylene glycol.
 6. A polyurethaneaccording to claim 1 wherein the dimer fatty acid content of thepolyurethane is in the range from 10 to 40% by weight.
 7. A polyurethaneaccording to claim 1 wherein the 1,4:3,6 dianhydrohexitol, preferablyisosorbide, content of the polyurethane is in the range from 2 to 10% byweight.
 8. A polyurethane according to claim 1 wherein the greenstrength value is greater than 50 kPa after 1 minute, and/or greaterthan 200 kPa after 5 minutes, and/or greater than 300 kPa after 30minutes.
 9. A polyurethane according to claim 1 wherein the tensilestrength is in the range from 30 to 200 KGCM2 and/or the elongation atbreak is in the range from 250 to 550%.
 10. A process for preparing apolyurethane which comprises (i) reacting a polyisocyanate with apolyester formed from a dimer fatty acid and/or dimer fatty diol, toform an isocyanate-terminated prepolymer, and (ii) reacting theprepolymer with a chain extender, wherein the polyester is additionallyformed from a 1,4:3,6 dianhydrohexitol and/or the chain extendercomprises a 1,4:3,6 dianhydrohexitol.
 11. A hotmelt adhesive comprisinga polyurethane obtainable by reacting a polyisocyanate, a polyesterformed from a dimer fatty acid and/or dimer fatty diol, and a chainextender, wherein the polyester is additionally formed from a 1,4:3,6dianhydrohexitol and/or the chain extender comprises a 1,4:3,6dianhydrohexitol.